The use of polymeric particles, including colloidal particles, either magnetic or non-magnetic, to bind a compound has long been known and used in industrial and laboratory procedures. For example, spherical polystyrene-divinylbenzene particles, Merrifield resins, were among the earliest and most widely used modern substrate particles. These particles found great utility in organic synthesis, in catalysis for heterogenizing homogeneous catalysts and in the biotechnical arts. Since these early particles were fairly large, they could easily be separated by filtration. However, in some fields, and particularly in the biochemical arts, it is desirable to use colloidal sized particles because the material being used is scarce, expensive, or is to be used in a procedure where larger particles cannot be used. When particles are of colloidal size, however, their separation from liquid medium can become a lengthy and difficult process because the colloidal particles tend to coat and plug the pores of filters. Consequently, magnetic colloidal particles are frequently used to avoid these difficulties.
The use of magnetic particles, specifically magnetic particles having a polymeric coating, has found great utility because such particles can be magnetically gathered to one side of a reaction vessel and the bulk of the reaction medium simply decanted. [As used herein, the words "particle" or "particulate substrate" or variation thereof encompasses spheres, spheroids, beads, and other shapes as well. These words are used interchangeably herein unless otherwise specified.] Magnetic particles are particularly useful in biological applications, especially where enzymes, antibodies and other substances are bound to the surface coating of the particles. The bound enzymes, antibodies or other substances may be used to capture a specific material from a sample in order to concentrate and analyze it, or to capture undesirable material from a sample, leaving the desired material in the sample for further use.
Practical considerations limit the usefulness of most polymer coated magnetic particles in medical and biological applications. Important factors for consideration are uniform particle size and shape, the need for the test reagent or sample to be tightly bound to the particle, the biodegradability of the particle or particle coating so that a test sample can be recovered, and the effect that the particle or particle coating will have on the test results when a sample is tested at various concentrations or in different mediums such as serum, whole blood or water to determine a standard curve or to assay the concentration of a substance in a test sample. Numerous types of particles, magnetic and non-magnetic, have been described in the patent and technical literature, and many are commercially available from sources well known to those skilled in the art. Examples of magnetic and non-magnetic particles may be found in U.S. Pat. Nos. 5,169,754; 5,248,772 and 5,527,713 (the '754, '772 and '713 patents, all to Simian et al.), and the references cited therein, all of which are incorporated herein by reference. While commercially available particles and the particles described in these patents have been found to have utility in biological applications, particularly in immunological assays, such particles are also known to exhibit a "matrix effect" which places a severe limitation on the utility of such particles.
A "matrix effect" results when a sample is assayed in varying sample media, for example, serum, whole blood and water, or at varying concentrations in a single medium. Assays results can exhibit a matrix shift or a measurement bias dependent on the solution medium and/or concentration. In particular, the matrix effect results when particles are used to remove an excess of a reagent so that the remaining solution can be assayed for a particular species. Frequently, additional substances, often of an unknown nature, are also removed by the particles. This removal produces the variation or matrix effect and makes it extremely difficult to reproduce assay results or correlate the test or assay results on a day-to-day basis or from assay-to-assay performed on the same day
The particles presently available have been found to exhibit this matrix effect. Consequently, it would be advantageous to have particles which minimize the matrix effect. The type of assays which would benefit from such a matrix effect minimizing substrate are generally known to those skilled in the art and examples can be found in U.S. Pat. Nos. 4,661,408 (the '408 patent) and 5,151,348 (the '348 patent), both to Lau et al.
Accordingly, it is an object of the invention to provide magnetic and non-magnetic particles suitable for use in immunological assays and compatible with biological substances.
It is a further object of the invention to provide magnetic and non-magnetic particles which, when used in immunological assays, minimize the matrix effect by selectively removing from a solution medium the species desired to be removed.
It is also an object of the invention to provide magnetic or non-magnetic particles suitable for use in immunological assays with antibodies, enzymes and similar biochemical substances.